The present invention is related to an improved ceramic foam filter. More particularly, the present invention is related to a ceramic foam filter for molten iron with sufficient mechanical properties for use in capturing and retaining high FeO liquid slags, other metal oxide slags, and other entrained particulates.
A significant portion of iron foundries use pressed or extruded mullite strainers. A ceramic foam filter is expected to perform better than a strainer due to its tortuous fluid flow path, but mullite strainers perform better than would be expected in ductile iron applications considering the strainers straight-through flow configuration. It is hypothesized that liquid slag in the molten ductile iron wets the surface of mullite better than it wets SiC. Mullite filters may better retain liquid slag during filtration but mullite foam filters for foundry applications have been unavailable.
Slag retention is difficult in strainer-type filters. Liquid inclusions, even though they readily wet the mullite strainer material, are very easily deformable by the flowing molten iron and simply flow down the filter wall to the exit. Oftentimes strainers act to coalesce numerous small slag inclusions (non critical size) and release them back into the flowing iron as a larger inclusion.
The state of the art for silica-bonded SiC foam filters used for molten iron filtration is described in U.S. Pat. No. 6,663,776. The filter described therein produces the highest high temperature strength commercially known for this particular type of filter. There are many companies that manufacture silica-bonded SiC filters for the iron industry because it is fairly easy to make a relatively robust filter of this type. During firing, the SiC grain in the body is believed to partially oxidize to silica glass. The silica glass allows the SiC grain to bond well with the silica binder matrix thereby creating a relatively robust foam. In the pouring of iron castings, particularly ductile iron, a high FeO liquid slag is formed. The high FeO slag does not wet SiC foam filters due to a carbothermic reaction between the carbon constituent of SiC grain and the graphite impurities in the SiC. The FeO slag reacts and CO gas forms at the slag-filter interface, preventing the slag from wetting and adhering to the filter.
Substituting SiC with mullite has proven to be elusive and proper design of the binder for formulation of a filter with the appropriate mechanical properties at both room and high temperature has not been previously discovered. Thus, one could fairly easily make an acceptable SiC filter even when using a substandard matrix design, but not with mullite.
A mullite filter with sufficient strength for liquid iron filtration has heretofore alluded researchers.